Zinc is a widely useful metal and its chemical compounds have a number of important applications. It is recovered from both oxidic and sulfidic ores. Another source of zinc is scrap. As zinc diecastings are important in a number of applications, and in particular as components of automobiles, a large amount of zinc metal-containing scrap is generated.
In contrast to copper and lead where a large portion of this scrap is recycled back to primary products, in the case of zinc only a small portion is recycled. An economic means of producing high purity metal from scrap is needed.
In the recovery of zinc from these materials, and in particular from sulfidic ores, it is common to roast the materials, producing noxious gases which are not totally recovered. This problem prompted a desire to develop alternate means, particularly hydrometallurgical means, which do not create the problems associated with zinc smelting. Aqueous electrolysis is one of these methods. However, this method is made expensive by the poor current efficiencies of the cells and by the fact that the zinc sulfate solutions commonly used for aqueous electrolysis are relatively poor electrical conductors. This method is also expensive in that zinc is plated on cathode sheets from which it must be stripped, washed, dried and melted.
Fused salt electrolysis is another method for recovering elemental zinc, but unless the starting material is anhydrous zinc chloride, moisture present in the feed will cause current inefficiency and carbon anode consumption. Prior methods for the production of anhydrous zinc chloride have been expensive and cumbersome.
Another problem associated with the use of anhydrous zinc chloride is caused by the extreme hygroscopic nature of this material. Accordingly, a simple hydrometallurgical process for producing anhydrous zinc chloride is required. This invention provides such a process and also produces an intermediate zinc chloride ammine compound which is more stable than anhydrous zinc chloride and may be washed and stored without absorbing moisture but is easily decomposed by heating to form anhydrous zinc chloride.